Recently, development of bottles, containers, sheets, and films using a biodegradable resin has been progressed in resin molded articles for use in food products, from a purpose of environmental loading reduction. The biodegradable resins usable for this application include polylactic acid, but there is a problem that polylactic acid resin is lower in various physical properties than resins which have been conventionally used. Thus, polylactic acid resin sheets having improved sliding property, impact resistance, or heat resistance by adding a biodegradable aliphatic polyester to the polylactic acid polymer, are proposed (See, Patent Documents 1 to 3).
For example, a film described in Patent Document 1 is obtained by adding a biodegradable aliphatic polyester having a glass transition temperature of 0° C. or less to a polylactic acid polymer to improve the impact resistance. In a sheet described in Patent Document 2, the improvement of the sliding property is attempted by adding a biodegradable aliphatic polyester having a glass transition temperature of 0° C. or less to a polylactic acid polymer having a composition ratio of L-lactic acid and D-lactic acid of 100:0 to 94:6 or 6:94 to 0:100 in a predetermined mixing ratio; at least uniaxially stretching the resin mixture; and subjecting it to a heat-treatment.
In addition, according to a method for molding a biodegradable sheet described in Patent Document 3, to a polylactic acid resin is added a polyester having a glass transition temperature of 0° C. or less and a melting point higher than a glass transition temperature of the polylactic acid resin and lower than a melting point of the polylactic acid resin in an predetermined mixing ratio. Then, a sheet composed of such a composition is pre-crystallized by bringing the sheet into contact with a cast roll, and molded at a temperature of the melting point of the aliphatic polyester or more, and less than the melting point of the polylactic acid resin, thereby improving the heat resistance of the sheet.
On the other hand, polylactic acid resin molded articles have the defect of having very poor barrier properties against a gas such as oxygen or carbon dioxide, and steam. Specifically, an oxygen-permeability of a polylactic acid film is about 5 times lower than those of aromatic polyester films which have been conventionally used, at 23° C. under a relative humidity of 80%, and a steam-barrier property thereof is about 10 times lower at 40° C. under a relative humidity of 90%. It is difficult to apply the molded articles having poor barrier properties as described above to materials for wrapping food and containers for cosmetics, and therefore, their possible applications are limited.
For imparting the barrier properties to the polylactic acid resin molded article, for example, a method in which a resin having a good barrier property such as an ethylene-vinyl alcohol copolymer is laminated therewith can be considered. Such a barrier material, however, has the defect of having poor biodegradability. Therefore, a multi-layer film in which polyglycolic acid having biodegradability is used as a barrier material has hitherto been proposed (see, Patent Document 4). The multi-layer film described in Patent Document 4 is a polyglycolic acid film on which a polylactic acid film(s) is/are directly laminated on its one or both surfaces, which is formed by co-extruding the polyglycolic acid and the polylactic acid to obtain a multi-layer sheet, and biaxially stretching the sheet under pre-determined conditions.